As generally known in the art, a piston for an internal combustion engine performs a role of compressing air provided to a cylinder through reciprocation within the cylinder, and transferring expansion pressure caused by the air and fuel combustion, to a connecting rod.
The piston is a component constituting a part of a combustion chamber for combustion in an engine, and includes a head part subject to high temperature and high pressure of exploded gas, a ring part, and a skirt part. The ring part has a piston ring made of special cast iron, which is put in 2 to 4 grooves dug in the circumference of the piston. The skirt part is subject to a side thrust, and has an outer diameter somewhat smaller than an inner diameter of the cylinder, against thermal expansion. The piston ring not only performs a role of maintaining the airtightness between the piston and the cylinder, but also scrapes off engine oil (or lubricant oil) on the cylinder.
While the piston compresses air, an exhaust gas is generated by combustion through fuel injection. The exhaust gas may move into a crank case through the lateral surface of the piston. On the other hand, the engine oil within the crank case may be moved to the combustion chamber and then combusted. The exhaust gas moving from the combustion chamber into the crank case is referred to as a blow-by gas, and the consumed amount of the engine oil moved from the inside of the crank case to the combustion chamber is referred to as an oil consumption amount.
Although the piston ring or the engine oil seals up between a cylinder wall and the piston, a gap between the cylinder wall and the piston cannot help but exist. Through such a gap, a trace amount of mixed gas is leaked into the crank case during compression stroke of the engine. The blow-by gas discharged to the crank case is transferred to a head cover.
When the blow-by gas stays within the crank case, the pressure within a crank is increased. This increases oil leakage at respective sealing portions, and deteriorates the engine oil. For this reason, the blow-by gas passes through an oil separator. Then, the oil is collected by the oil separator, while the blow-by gas, from which the oil has been separated, is sent again to an intake manifold, and is then supplied and re-combusted, together with the intake air, in the combustion chamber, or simply discharged to the atmosphere.
An increase in both the blow-by gas and the oil consumption has a bad effect on the performance of the engine. When the oil consumption increases, a financial burden is increased, thereby causing dissatisfaction of consumers. Also, when the blow-by gas is instable, oil consumption increases due to instable behavior of the piston ring, or the performance of a closed crankcase ventilation (CCV) system becomes low due to instability of the internal pressure of the crank case. Especially, in a case where a filter-type CCV is used, such instability has an effect on the life of a filter, which increases a financial burden of the consumers, thereby causing dissatisfaction of them.
FIG. 1 shows a conventional piston of a diesel engine. Referring to FIG. 1, the conventional piston for the diesel engine includes an upper ring 102, a middle ring 112, and an oil ring 122, provided therewithin. Between the upper ring 102 and the middle ring 112, a ring-shaped groove 140 is formed, and there are small gaps between the upper/lower ends of the ring-shaped groove 140, and a liner. FIGS. 2 and 3 show graphs illustrating pressure fluctuations of a blow-by gas in 8-liter grade and 11-liter grade diesel engines which employ the piston shown in FIG. 1.
Referring to FIGS. 2 and 3, although a conventional piston for a diesel engine has the ring-shaped groove 140 between the upper ring 102 and the middle ring 112, the gaps between the upper/lower ends of the ring-shaped groove 140, and the liner are the same as each other, and relatively small. Thus, the blow-by gas coming out from the combustion chamber via the upper ring 102 stays between the upper ring 102 and the middle ring 112, and forms a pressure, which destabilizes the behaviors of the upper ring 102 and the middle ring 112. This destabilizes the behavior of the blow-by gas coming into the crank case via the middle ring 112, thereby causing non-uniformity of a change in a pressure diagram of the blow-by gas, and increasing the consumption of oil.
Meanwhile, as another example of a conventional technology, Japanese Patent Publication No. 2001-214804 discloses a technology on a piston having a step-shaped ring-shaped groove formed between an upper ring and an oil ring. In the piston according to the conventional technology, since the ring-shaped groove between the upper ring and the oil ring is formed in a step shape with a wider upper space and a narrower lower space, there is a problem in that oil staying in the upper portion of the ring-shaped groove cannot smoothly flow down to the oil ring by being subjected to resistance of the height of the step. Also, such a structure results in an increase in oil consumption, and a decrease in capability of reducing a blow-by gas.
As a further example of a conventional technology, Japanese Patent Publication No. 2001-214805 discloses a technology on a piston having a wedge-shaped ring-shaped groove formed between an upper ring, and an oil ring. Since the piston according to the conventional technology has a structure when the ring-shaped groove between the upper ring and the oil ring is formed into a wedge shape with a wider upper space and a narrower lower space, it is impossible to secure a sufficient amount of oil in the upper portion of the ring-shaped groove. Thus, there is a problem in that oil in the upper portion cannot smoothly flow down to the oil ring. Also, in a case where such a wedge-shaped ring-shaped groove is designed with a large size, an area between from the end of the ring-shaped groove to the oil ring becomes smaller, thereby reducing the controllability of a blow-by gas. Accordingly such a structure results in an increase in oil consumption, or a decrease in capability of controlling a blow-by gas.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.